The present invention relates generally to halftone screening processes and, more particularly, to a system and method for a tone-dependent multi-frequency halftone screen.
Halftone screening processes function to transform a continuous tone image into a binary image that is to be rendered and perceived by an observer as an original continuous tone image. Halftone screening processes typically apply a halftone screens to a continuous tone image. The result is binary image that appears to be made up of patterns or groups of individual black or white printer device dots. Each pattern or group has a proportion and arrangement of black and white dots so as to render, from a distance, an impression of a selected level of gray. Thus, when a halftone image is observed from a typical viewing distance, it appears as an original, continuous tone image. Currently, halftone screening processes are used in printing and display devices such as laser printers, dot matrix printers, inkjet printers; and the like.
Halftoning is necessary because many output devices are not capable of producing all of the shades or colors often contained in continuous tone images. For example, a laser printer may have only one color of ink; typically, black. There are no grays. Halftoning permits the appearance of a number of shades of gray.
Halftone screens are defined by screen frequencies, typically measured in lines per unit of length, such as lines per inch (lpi). Thus, screen frequency is often represented by a grid. Each square in the grid then represents a halftone cell capable of holding a halftone dot pattern. Higher screen frequencies produce finer halftone screens, while lower screen frequencies produce coarser halftone screens. Further, multiple screen frequencies are represented by multiple grid or halftone screens.
To convert a continuous tone image into a halftone image, a halftone screen or grid is typically superimposed on the continuous tone image. Each halftone cell in the halftone screen or grid is then assigned a different sized dot to represent the continuous tone image data for that halftone cell. Again, when all of the dots are viewed together at a normal viewing distance, the dots appear as the original continuous tone image.
The size of the halftone cells is determined by the interaction of the selected screen frequency with a printer's device resolution. The word “printer” refers to any mechanism known in the art suitably capable of making marks on a physical substrate. A- printer creates an electronic version of the halftone screen, while screening software r applies a selected dot pattern to the electronic image. The image recorder resolution setting reflects the image recorder's ability to place device dots close together. For example, a device dot is created by an image recorder laser beam when it is focused on a point on a print drum. When the halftone image is printed, the area on the piece of paper corresponding to the area exposed to the laser beam is black. The closer together the laser can place the dots and the smaller the dot diameter, the higher the halftone resolution. Thus, groups of device dots composing the grid are commonly referred to as “printer dots,” and further, printer device resolution is measured in device dots per inch (dpi), and is represented by a grid.
When the halftone grid is laid over the resolution grid, each halftone cell is filled with device dots. Groups of device dots form halftone dots. Thus, each of the halftone cells in the previous example is comprised of many device dots that are created by the printer's laser, forming the halftone dots. Each of these device dots created by the image recorder is selectively turned on, producing a final output, e.g., gray scale, or turned off, producing no output or white.
The group of device dots within a halftone cell produces a halftone dot of a specific size and shape. For example, if the halftone dot needs to be bigger, the printer's laser turns on more device dots. Similarly, if the halftone dot needs to be smaller, the printer's laser turns on fewer device dots. To create different shapes, the printer's laser turns the device dots on in different sequences. Each sequence is determined by a mathematical equation referred to as a spot function or, more commonly, by a sequence of numbers referred to as a threshold array. Different spot functions and array sequences exist for each dot shape. Common shapes include round, diamond, square and elliptical.
Halftone names can be confusing. For example, there are two types of square-shaped halftones. In one of these, the halftone dots are shaped like squares all the way through the tint or grey scale. In the other, the halftone dots start out shaped like circles, grow to square shapes in the mid-tones, and then become circular again. In addition, for example, different manufacturers of printing devices use different spot functions to create halftone dots. Thus, not every manufacturer's round or square dots, for example, grow in exactly the same way.
One print standard, commonly referred to as PostScript Adobe Systems, has emerged which includes a system for handling gray levels. PostScript requires at least 256 levels of grey to properly reproduce a continuous tone image. Because of this requirement, manufacturers have adopted 256 grey levels as a de facto standard.
Generally, it is desirable to increase the screen frequency and to increase the number of tone levels to provide a halftone image that more closely resembles the original continuous tone image. However, there is a trade-off between higher screen frequencies and the number of tone levels. Higher screen frequencies, by virtue of containing more halftone cells, produce finer screens that can capture more detail of the original continuous tone image. Therefore, because the resolution remains constant, the more halftone cells there are, the fewer device dots each halftone cell can contain. Furthermore, as the number of device dots within each halftone cell decreases, so does the number of tone levels each halftone cell can reproduce.
Thus, there exists a need for a system and method that provides increased resolution while also providing an increase in the number of tonal levels.